This invention relates to a method of manufacturing a glass optical element, such as a glass lens, by pressing without necessity of a grinding or a polishing process after pressing. More particularly, this invention relates to a forming method which can manufacture a glass optical element with high optical quality, with high productivity, and with a short cycle, time.
Heretofore, various methods have been proposed which manufacture a glass optical element only by pressing or press forming a glass preform of a raw glass material within a forming mold. In this event, the forming mold should have high surface accuracy and smooth surface roughness to obtain a final glass optical element.
Coventionally, disclosure is made in U.S. Pat. No. 3,833,347 (the first reference 1) about a method which is called an isothermal pressing method and which comprises the steps of preliminarily disposing a glass material in a forming mold, heating the glass material together with the mold to put both the glass material and the mold into an isothermal state and to thereby soften the glass material, and press forming the glass material under a pressure in the isothermal state. Thereafter, the glass material pressed is cooled to a temperature lower than the glass transition point thereof with the pressure kept intact.
This isothermal press method does not cause any sink mark to occur in the glass optical element and serves to form precise surfaces on the glass optical element with the isothermal state kept during the press forming between the glass material and the forming mold. This shows that the glass material is kept at a temperature substantially equal to that of the forming surfaces.
This method, however, is very low in productivity because it takes a very long time to raise up the temperature before the press forming step and to cool the glass optical element after the press forming process. This shows that a cycle time of the whole process necessary for manufacturing the glass optical element becomes long. Accordingly, the glass material is contacted with the forming surfaces of the forming mold for a long time. Such a long time contact between the softened glass material and the mold surfaces causes reaction to occur between the glass material and the mold surfaces and is liable to bring about fusion and opacfication onto the glass material. Moreover, this results in shortening a life time of the forming mold.
In order to overcome these disadvantages, consideration has been made about a non-isothermal press method. In this method, a glass preform previously softened is introduced into a forming mold which is kept at a relatively low temperature. Specifically, JP-A-59-203,732 (the second reference 2) discloses a non-isothermal press method which comprises the steps of softening a glass material to a temperature which corresponds to a viscosity in the range of 105.5-107 poises and press forming the softened glass material by the use of a forming mold kept at a temperature which is lower than the temperature of the glass material by 100xc2x0 C., namely, lower than the transition point of the glass by about 20xc2x0 C. In this method, the glass material is softened and conveyed or transported with the glass material held in a holder. However, the glass material is too soft and is therefore often deformed while it is being held in the holder and being transported and/or adhered to the holder.
Furthermore, JP-A-62-27,334 (the third reference 3) discloses a method which comprises the steps of heating a glass material to a temperature which corresponds to the viscosity between 106 and 108 poises and which is comparatively low in comparison with the second reference 2. On the other hand, a forming mold is kept at a temperature between a transformation temperature (Tg) and (Tg-200xc2x0 C.). Under the circumstances, the press forming is carried out by the use of the forming mold heated above. This method, however, is disadvantageous in that sink mark and wrinkles objectionably take place on the glass optical element. This is because the temperature of the forming mold is very low and, as a result, the glass material is quickly cooled within the forming mold during the press forming process. Therefore, a desirable surface accuracy of the glass optical element can not be accomplished with this method. Moreover, when a thin-lens is manufactured which has a predetermined thickness, it is difficult to press the glass material to the predetermined thickness because the glass material is not kept at a comparatively hard state within the glass mold and can not be therefore pressed to the predetermined thickness.
In order to avoid the defects and the disadvantages of the above-mentioned references, typical temperature conditions are disclosed in JP-A-07-10,556 (the fourth reference). In the fourth reference 4, a glass material is heated to a temperature which corresponds to the viscosity thereof between 107 and 109 poises and is introduced into a mold which is preheated to a temperature corresponding to the viscosity of the glass material between 1010 and 1012 poises. Thereafter, the glass material is pressed within the forming mold and then cooled together with the forming mold to a temperature corresponding to 1013 poises. Subsequently, the pressure is relieved from the forming mold.
This method, however, brings about adhesion of the pressed glass material to an upper or a lower die of the forming mold, although the pressed glass material is not fused onto the forming mold on releasing the pressed glass material from the forming mold. Accordingly, this method must be practically cooled to a very low temperature prior to separation or release of the forming mold. This results in lengthening a cycle time for manufacturing the pressed glass material.
Incidentally, JP-A-02-59,449 (the fifth reference 5) discloses a method which prevent a shaped glass article from being adhered to a forming mold. With this method, the forming mold is opened or separated into an upper die and a lower die which are kept at different temperatures. Therefore, a pressed glass material is taken out from the forming mold after the pressed glass material is cooled to the temperature of the forming mold mentioned above.
More specifically, the upper and the lower dies are at first kept at the same temperature while a glass material is also preheated so that it has a viscosity enough for press forming. Subsequently, the glass material is introduced within the upper and the lower dies and is pressed under a pressure. Thereafter, the upper and the lower dies are gradually cooled together and the lower die alone is further cooled by blowing a cooling gas onto the lower die. The temperature of the resultant lower die is lower than that of the upper die. Thus, a temperature difference is caused to occur between the upper and the lower dies. Under the circumstances, the upper and the lower dies are opened or separated from each other to take out the pressed glass material.
This method is however disadvantageous in that cracks or the like are liable to be caused to occur in the pressed glass material and a cycle time becomes long. In addition, a desirable surface accuracy can not be accomplished by this method.
In JP-A-05-70,154 (the sixth reference 6) also, a temperature difference is given to upper and lower dies on separating the upper and the lower dies. Specifically, pressed glass material is released from one of the dies kept at a low temperature and is thereafter released from the other die kept at a high temperature. However, the pressed glass material is often adhered to the other die kept at the high temperature even when it is released from the one die. Therefore, a specific tool must be prepared so as to release the pressed glass material from both of the dies. In addition, when the pressed glass material is adhered to the upper die, it might be dropped and broken in the course of separation or release.
In the previous Japanese patent application No.
Hei 7-259015, the present inventors have already proposed a method of manufacturing a pressed glass element. More particularly, the method comprises the steps of press forming a thermally softened glass material, such as a glass preform. To this end, the glass material is heated prior to the press forming to a temperature which falls within a specific range while the forming mold is also heated to a temperature which falls within a particular range. In this situation, the glass material is pressed by the forming mold within the specific range and is released form the forming mold after the forming mold becomes a temperature lower than the specific range. With this method, it is possible to manufacture the pressed glass element of an excellent property within a short cycle time. In addition, the pressed glass element has a good surface accuracy.
However, transient adhesion of the pressed glass element to the upper die takes place sometimes on separating the forming mold into the upper and the lower dies. Therefore, there is a problem that separating the upper and the lower dies is unstable. Such unstable separation of the forming mold makes it difficult to shorten a cycle time on successively mass-producing the glass optical elements.
It is, therefore, an object of this invention to provide a method which is capable of successively manufacturing a great number of glass optical elements, such as lenses, by press forming a thermally softened glass material, such as glass preform.
It is another object of this invention to provide a method of the type described, which is suitable for mass-producing the glass optical elements.
It is still another object of this invention to provide a method of the type described, which is capable of avoiding adhesion of the glass optical element to a forming mold.
It is yet another object of this invention to provide a method of the type described, which is capable of shortening a cycle time necessary for manufacturing the glass optical element.
It is another object of this invention to provide a method of the type described, which is capable of manufacturing the glass optical element having an excellent surface accuracy without any sink mark, deformation, and the like.
It is a further object of this invention to provide a method of the type described, which is capable of precisely manufacturing a glass optical lens with a thin-thickness at an edge portion.
According to the inventors""experimental studies, it has been found out that the problems of the references mentioned above can be solved by preheating upper and lower dies of a forming mold to a first predetermined temperature and a second predetermined temperature higher than the first predetermined temperature and by pressing a glass material softened and kept at a third predetermined temperature higher than the second predetermined temperature.
According to this invention, there is provided a method for forming a glass optical element by press forming a thermally softened glass material with a preheated forming mold composed of an upper and a lower die, characterized by setting the temperature for heating the glass material at a level equivalent to less than 109 poises of the viscosity of the glass material, setting the temperature for preheating the upper die at a level equivalent to 109.5-1012 poises of the viscosity of the glass material, setting the temperature for preheating the lower die at a level equivalent to 109-1011.5 poises of the viscosity of the glass material, and further keeping the preheating temperature of the upper die below the preheating temperature of the lower die, subjecting the thermally softened glass material to initial pressing in the preheated forming mold under a pressure in the range of 20-350 kg/cm2 for a period in the range of 1-30 seconds, cooling the neighborhood of the forming surfaces of the upper and the lower dies at a speed of 20-180xc2x0 C., simultaneously with the start of the initial pressing, during the process of the initial pressing, or after the completion of the initial pressing, relieving the forming mold of the pressure after the temperatures in the neighborhood of the forming surfaces of the upper and the lower dies have fallen below a level equivalent to 1012.5 poises of the viscosity of the glass material, and releasing a shaped article of glass from the forming mold.
According to this invention, there is also provided a method of forming a glass optical element by pressing a thermally softened glass material by the use of a preheated forming mold composed of an upper and a lower die, the method comprising the steps of heating a glass material at a temperature which corresponds to less than 109 poises in the viscosity of the glass material, to form the thermally softened glass material; preheating the upper die at a first temperature which corresponds to 109.5-1012 poises in the viscosity of the glass material; preheating the lower die at a second temperature which corresponds to 109-1011.5 poises in the viscosity of the glass material and which is higher than the first temperature; subjecting the thermally softened glass material to initial pressing within the preheated upper and lower dies under a pressure between 20 and 350 kg/cm2 for a period between 1 and 30 seconds; cooling the upper and the lower dies so that portions adjacent to forming surfaces of the upper and the lower dies are cooled at a speed of 20-180xc2x0 C., simultaneously with the start of the initial pressing, during the process of the initial pressing, or after the completion of the initial pressing; relieving the forming mold of the pressure after the temperatures in the portions adjacent to the forming surfaces of the upper and the lower dies have fallen below a temperature corresponding to 1012.5 poises in the viscosity of the glass material; and releasing a shaped article of glass as the glass optical element from the forming mold.